The present embodiments relate to wireless communication systems and, more particularly, to a method and apparatus for interference estimation in a Long Term Evolution (LTE) wireless receiver.
Conventional cellular communication systems operate in a point-to-point single-cell transmission fashion where a user terminal or equipment (UE) is uniquely connected to and served by a single cellular base station (eNB or eNodeB) at a given time. An example of such a system is the 3GPP Long Term Evolution (LTE Release-8). Advanced cellular systems are intended to further improve the data rate and performance by adopting multi-point-to-point or coordinated multi-point (CoMP) communication where multiple base stations can cooperatively design the downlink transmission to serve a UE at the same time. An example of such a system is the 3GPP LTE-Advanced system (Release-10 and beyond). This greatly improves received signal strength at the UE by transmitting the same signal to each UE from different base stations. This is particularly beneficial for cell edge UEs that observe strong interference from neighboring base stations.
FIG. 1 shows an exemplary wireless telecommunications network having cells A and B. The illustrative telecommunications network includes base stations 100 in cell A and 110 in cell B, though in operation, a telecommunications network necessarily includes many more base stations. Base station 100 is synchronized with UEs 102 and 106 and communicates over respective wireless channels 104 and 108. Likewise, base station 110 is synchronized with UE 112 and communicates over wireless channel 114. Because each UE is synchronized with its respective base station, intra-cell interference is not a significant problem. For example, UEs 102 and 106 do not significantly interfere with each other or with base station 100. Base stations 100 and 110, however, are not synchronized. Therefore, UE 112 is not synchronized with either UE 102 or 106. This lack of synchronization causes significant inter-cell interference for UEs near a cell boundary. For example, UE 106 primarily communicates with base station 100 over channel 108. Thus, uplink transmission from UE 106 to base station 100 produces significant inter-cell interference 116 at base station 110. Likewise, downlink transmission from base station 110 to UE 112 produces significant inter-cell interference 116 at UE 106.
Turning now to FIG. 2, there is a diagram of a subframe 200 having a Physical Resource Block (PRB) pair. The eNB may configure 1, 2, 4, or 8 PRB pairs for communication with the UE. However, each PRB pair is a replica, and only one PRB pair is shown for the purpose of explanation. Each column of the diagram of the subframe corresponds to 12 subcarriers or tones in an OFDM symbol. There are 14 OFDM symbols in the subframe with a normal cyclic prefix (CP). The 3 OFDM symbols on the left side of the subframe include resource elements (REs) for transmission of a legacy physical downlink control channel (PDCCH) and legacy cell-specific reference signals (CRS). These 3 OFDM symbols are necessary for backwards compatibility with previous wireless standards. The 11 OFDM symbols on the right include resource elements (REs) for transmission of an enhanced physical downlink control channel (EPDCCH), and demodulation reference signals (DMRS) or pilot signals, as well as cell-specific reference signals (CRS) and orphan or unused REs. Orphan REs may exist because the UE always assumes that 24 REs are reserved for DMRS transmission in a PRB pair configured for EPDCCH transmission.
A cause of inter-cell interference is that both base stations use the same subcarriers or tones for each PRB with reuse 1. This means that the base station assumes that all 12 subcarriers are available for each PRB and is especially problematic in areas of dense deployment. If only a portion of the subcarriers were allocated to each base station, inter-cell interference would be reduced at the expense of bandwidth and throughput. Several attempts to reduce inter-cell interference through inter-cell interference coordination (ICIC) technology have been developed. For example, Kimura et al., “Inter-Cell Interference Coordination (ICIC) Technology,” Fujitsu Sci. Tech. J., Vol. 48, No. 1, pp. 89-94 (January 2012), have developed a method of fractional frequency reuse (FFR) to allocate different frequencies to UEs near a cell boundary. Others, such as Xing (U.S. Pub. No. 2014/0078922) employ a spreading code for adjacent cells to identify and remove interfering signals. Other methods rely on channel estimation as determined from known pilot signals. For example, Dua et al. (U.S. Pub. No. 2014/0016689) programs an equalizer by estimating a channel impulse response (CIR) and determining noise and power estimates based on the CIR. Equalizer inputs of a covariance matrix are adjusted based on these noise power estimates. A disadvantage of this method, however, is that errors in channel estimation are considered interference and noise. Moreover, in areas of dense deployment near cell boundaries, signal quality is degraded and channel estimation errors are significant.
While the preceding approaches provide steady improvements in wireless communications, the present inventor has recognized that still further improvements in interference detection are possible. Accordingly, the preferred embodiments described below are directed toward this as well as improving upon the prior art.